Some cutting tools for special applications or for extended life generally are of a compound type, in which a very hard cutting element, or cutter, of one material is held in a holder of softer, base material. Such a compound arrangement is used for flat tools, such as cutting tools for lathes, and round tools, such as drill bits, reamers, and end mills. In each case, a cutting element is secured to, the holder to present a cutting edge, a flank face, and a rake face to the workpiece to be machined. In the course of a cutting operation, the flank face is forced against and moves generally parallel to the workpiece surface. Examples of materials commonly used for cutters are tool steel, tungsten carbide, ceramic, sapphire, and diamond. Examples of materials commonly used as base material are steel, tungsten carbide, and ceramic. The flank face is generally considered to be the end face of a cutting tool which extends from the cutting edge in the general direction of the movement of the tool relative to the workpiece, with some deviation from this direction being provided for clearance purposes, however. The rake face is that face of the tool to the other side of the cutting edge from the flank face and is sometimes also referred to as the "cutting face."
For many applications diamond is considered to be the ultimate cutter material because of its extreme hardness and resistance to important types of chemical corrosion. The diamond may be in the form of diamond compact material, also referred to as polycrystalline diamond (PCD) made of diamond micro-crystallites held in a cobalt matrix. It may also be either natural or CVD diamond film, which are both even harder than PCD material and exhibit better performance for many applications because the cobalt binder of the PCD material increases the friction of the tool, reduces the chemical and thermal stability, and can also act as a contaminant for the Workpiece. Diamond film has an advantage over natural diamond for cutting tool applications in that it can be manufactured in geometries which are very difficult, if not impossible, to obtain with natural diamond. For example, it can be made in free-standing, polycrystalline, flat wafer form to be bonded to a holder as the cutter tip. It can also be deposited directly on the tool holder as either a thick film or a thin film. In this context a thin film is a film too thin to be free-standing and is usually on the order of about thirty microns or less in thickness.
Despite the outstanding properties of CVD diamond film for implementation as a cutter element, such material nevertheless does show wear after extensive use. The mechanism of failure is believed to be due to the generation on the rake face of tensile stresses which exceed the fracture strength of the diamond. As a result, the material breaks away at the flank face. Typically, this would happen when a hard lump in the workpiece material being cut hits the rake face very near the cutting edge, producing high local tensile stresses on the rake face generally in the plane of the rake face. At some flaw near the surface, these tensile stresses induce a crack which propagates downward and eventually breaks a small chip from the end of the tool.